The present invention relates to a capacitive pressure sensor and, more particularly, to a capacitive pressure sensor in which extraction electrodes made of molten solder are connected to a stationary electrode and movable electrode that constitute a capacitor element.
In a conventional capacitive pressure sensor, a thin wafer constituting a diaphragm and a thick wafer with a recess to form a base are adhered to each other, and the recess and diaphragm constitute a capacitor chamber for a capacitor element. Electrodes constituting the capacitor element are arranged in the capacitor chamber to oppose each other.
As shown in FIG. 9, a conventional capacitive pressure sensor 101 is comprised of a lower wafer 102, an upper wafer 103, extraction electrodes 104, a stationary electrode 105, a movable electrode 107, a reference electrode 109, a pad 106 for the stationary electrode 105, and pads 108 for the movable electrode 107 and reference electrode 109.
Both the lower and upper wafers 102 and 103 are substrates made of sapphire, silicon, glass, or alumina. The lower wafer 102 has a circular recessed capacitor forming portion 102a at its central portion excluding the periphery, and a plurality of pad forming portions 102b and 103b projecting outward from the outer circumference of the capacitor forming portion 102a. The upper wafer 103 is bonded to the periphery of the lower wafer 102 so as to cover the capacitor forming portion 102a and pad forming portions 102b and 103b. The upper wafer 103 constitutes a diaphragm as it is formed sufficiently thin such that it can be easily deflected in accordance with a change in external pressure.
As shown in FIG. 8, the circular movable electrode 107 is fixed in tight contact with the central portion on one surface of the diaphragm opposing the capacitor forming portion 102a, and the C-shaped reference electrode 109 is fixed in tight contact with the edge of the diaphragm to substantially surround the movable electrode 107. The circular stationary electrode 105 is fixed in tight contact with the lower wafer 102 to oppose the movable electrode 107 and reference electrode 109. The electrodes 105, 107, and 109 are connected to the extraction electrodes 104 that extend through the lower wafer 102. The movable electrode 107 and reference electrode 109 of the upper wafer 103 and the stationary electrode 105 of the lower wafer 102 oppose each other through a predetermined gap to constitute a capacitor element.
In this arrangement, when a diaphragm 103a is deflected by a pressure change, the movable electrode 107 is displaced accordingly to change the distance between the movable electrode 107 and stationary electrode 105. A change in capacitance between the stationary electrode 105 and movable electrode 107 is electrically detected to measure the pressure change indirectly. The reference electrode 109 is used to correct the capacitance detected between the stationary electrode 105 and movable electrode 107.
A method of manufacturing the capacitive pressure sensor described above will be briefly described. The lower and upper wafers 102 and 103 are prepared by processing a substrate made of sapphire or the like. Through holes 110 for forming the extraction electrodes 104 are formed in the lower wafer 102 by machining, a laser process, an ultrasonic process, or the like. A recess for the capacitor forming portion 102a is formed in the surface of the lower wafer 102 by dry etching.
A metal film is formed in the recess by vapor deposition, ion plating, sputtering, or the like, and is selectively etched to form the stationary electrode 105. The stationary electrode 105 is formed of a Pt/adhesion promoter film. To form the adhesion promoter film, Ti, V, Cr, Nb, Zr, Hf, Ta, or the like is used. Obviously, etching may not be performed and sputtering or the like may be performed through a shadow mask.
In the upper wafer 103, a metal film is formed by sputtering or the like on a substrate made of sapphire or the like, and is selectively etched to form the movable electrode 107, reference electrode 109, and pads 106 and 108. The pad 106 is formed of an Au/barrier film/adhesion promoter film. For example, Pt is used to form the barrier film, and Nb is used to form the adhesion promoter film. Obviously, instead of etching the metal film to form the electrodes, sputtering may be performed through a shadow mask to form the electrodes.
After that, the upper wafer 103 is adhered to the lower wafer 102, and the upper and lower upper wafers 103 and 102 are directly bonded to each other in an atmosphere with a temperature condition of 400xc2x0 C. to 1,300xc2x0 C. After bonding, molten solder 104a such as Snxe2x80x94Ag solder is filled in the through holes 110 in the lower wafer 102 to form the extraction electrodes 104. If the lower and upper wafers 102 and 103 are positioned in advance such that the through holes 110 and the pads 106 and 108 oppose each other, the molten solder 104a filled in the through holes 110 attaches to the pads 106 and 108 to make reliable electrical connection.
The conventional pressure sensor described above has several problems. More specifically, in the upper wafer 103 constituting the diaphragm, the surface where the movable electrode 107 and reference electrode 109 are to be formed and the surface to be bonded to the lower wafer 102 are located on the same plane. If defective electrode formation or wafer misalignment occurs, a misaligned electrode may interfere with bonding the wafers.
Generally, when the lower and upper wafers are fabricated from the same material (e.g., sapphire), they are often bonded to each other by direct bonding. Since direct bonding requires planarity and small surface roughness in the bonding surfaces, a misaligned electrode largely decreases the bonding strength of the wafers. For this reason, conventionally, electrodes and lead portions attaching to them must be sufficiently distant from the bonded portions of the wafers. This requires an extra space to interfere with downsizing the sensor.
When forming the extraction electrodes 104, the molten solder 104a can flow into the capacitor chamber through interconnections to short-circuit the electrodes with each other. In order to prevent this, conventionally, as shown in FIG. 9, a step xcex3 is formed to shorten the distance between the pads 106 and 108 and the openings of the through holes 110, so that the amount of solder that flows out is suppressed. With such a sharp step xcex3, however, it becomes difficult to form a metal film in a region xcex4 during sputtering or the like, causing defective interconnection formation.
It is an object of the present invention to provide a capacitive pressure sensor in which defective bonding can be prevented from being caused depending on the material of a misaligned electrode when base members respectively having electrodes are to be bonded to each other.
It is another object of the present invention to provide a capacitive pressure sensor in which molten solder which forms extraction electrodes can be prevented from flowing into a capacitor chamber.
In order to achieve the above objects, according to the present invention, there is provided a capacitive pressure sensor comprising a first base member with a first main recess and a first sub-recess communicating with the first main recess, a second base member with a second main recess constituting a capacitor chamber together with the first main recess, and a second sub-recess communicating with the second main recess, the second main recess having a bottom surface that constitutes a diaphragm, a stationary electrode formed on a bottom surface of the first main recess, a first pad formed on a bottom surface of the first sub-recess and connected to the stationary electrode through a first interconnection, a movable electrode formed on a bottom surface of the second main recess to oppose the stationary electrode, a second pad formed in the second sub-recess and connected to the movable electrode through a second interconnection, and a plurality of extraction electrodes connected to the first and second pads and extracted outside through through holes formed in the first base member, wherein the first and second base members are bonded to each other such that the first and second sub-recesses are covered with bonding surfaces of the second and first base members.